Satellite positioning systems, such as Global Positioning System (GPS), are used to aid in navigation by providing absolute coordinates for a person, vehicle, or other object. Navigation using GPS is referred to as “GPS-aided” navigation. At times, due to signal blockage between GPS satellites and individual GPS receivers, the signal is not received at the GPS receivers. This happens when there is no line of sight between the GPS satellites and the individual GPS receivers, such as when the receivers are under a dense tree canopy, in storms or clouds, in valleys blocked by mountains, inside mines or caves, in urban areas surrounded by tall buildings, or inside buildings or other structures. During these times of limited or no available GPS signal, a person, vehicle, or other object is said to be under “GPS-denied” navigation.
GPS-denied navigation has received increasing interest in recent years. Some approaches to GPS-denied navigation have been recently developed. Some of these approaches utilize inertial measurement units (“IMU”) in conjunction with other sensors and an estimator, such as a Kalman filter, to estimate position, velocity, and attitude of a platform. One of these sensors can be a camera which can provide information regarding change in position and attitude.
Because monocular cameras cannot provide depth information unless depth estimation algorithms are used to estimate depth, a monocular camera not generally provide adequate information to calibrate the IMU. Therefore, when using only a monocular camera together with an IMU, position and velocity estimations will drift.
Drifts in velocity and/or attitude estimation can be periodically reduced to virtually zero if zero velocity updates and/or zero attitude rate updates are performed. Zero velocity update and/or zero attitude update events may be triggered automatically by logic that looks at the IMU outputs at any point in time. For low cost IMUs, zero velocity updates and/or zero attitude rate updates are not easy to reliably detect and trigger given the noisy nature of the sensor.